In an ignition device including an ignition coil, by, after feeding a primary coil with a primary current, cutting off the primary current at a predetermined ignition timing, a high discharge voltage is produced in a secondary coil causing an ignition plug connected to the secondary coil to produce an electric discharge between electrodes of the ignition plug. The discharge voltage and discharge energy produced in the secondary coil basically depend on an energization time for the primary coil.
In Patent Document-1, there is disclosed a technology in which in order to obtain an assured ignition by elongating the discharge period, a superimposed voltage produced by a different booster is fed to the ignition plug during the discharge period after the ignition timing. In this technology, after starting the discharge between the electrodes by the secondary voltage produced by the ignition coil, a discharge current is continued by the superimposed voltage and thus, much larger energy is given to an air/fuel mixture.
In general, the energization time for the primary coil that controls the discharge energy is determined by a rotation speed of the engine, and when the engine rotation speed is low, the energization time needed becomes long. However, in Patent Document-2, there is disclosed a technology in which in a higher load operation range, the energization time is increased and in a lower load operation range, the energization time is reduced.
Although feeding of the superimposed voltage like in the technology disclosed by Patent Document-1 is effective for improving ignition performance, the feeding has such a drawback that due to a heat generation of a superimposed voltage generation circuit in an ignition unit including the ignition coil, the ignition unit is subjected to a temperature increase. Particularly, in a higher engine rotation speed range, the temperature increase of the ignition unit is remarkable, and thus, in such higher engine rotation speed range, feeding of the superimposed voltage can't be used or it is necessary to provide the ignition unit with a high heat resistance.
Patent Document-2 shows only an example in which the energization time for the primary coil is changed between the higher load operation range and the lower load operation range, and the publication does not prepare any description on the temperature increase of the ignition unit.